In adults the body mass index (BMI) is assessed against fixed thresholds, but with children and teenagers things are different. A growing body changes its weight and height proportions from month to month, which is why a child’s BMI is interpreted using percentile charts that take age and sex into account. Here we explain how this works.
How is BMI calculated?
The formula is the same as for adults: body weight in kilograms is divided by height in metres squared (BMI = weight / height²). For example, a child weighing 20 kg and measuring 1.15 m has a BMI of 20 / (1.15 × 1.15) ≈ 15.1. The number alone, however, means nothing for a child without reference to age.
What are percentiles?
A percentile tells you what proportion of children of the same age and sex have a value lower than the child in question. If a child’s BMI sits on the 50th percentile, this means an exactly average result: half of their peers have a lower BMI and half have a higher one. A result on the 90th percentile means that 90 percent of peers have a lower BMI.
Norms for interpreting BMI in children
The following percentile ranges are most commonly used:
- Below the 5th percentile: underweight.
- From the 5th to the 85th percentile: healthy weight.
- From the 85th to the 95th percentile: overweight.
- Above the 95th percentile: obesity.
These thresholds differ from those for adults, where overweight is diagnosed from a BMI of 25 and obesity from 30. For children, rigid numbers do not apply.
Interpretation example
Suppose a 7-year-old boy with a BMI of 15.1. To assess the result, we refer it to the percentile chart for boys of this age:
- We read off the chart which percentile the value 15.1 lies on.
- If it falls on, say, the 50th percentile — the weight is healthy.
- If the same value of 15.1 applied to a 4-year-old, it could already lie higher, closer to the 85th percentile, because younger children have lower average BMIs.
The same numerical result therefore means something different depending on age — this is the heart of the difference between assessing BMI in children and in adults.
What to pay attention to?
The trend is often more important than a single measurement. If a child suddenly jumps up or down across several percentile channels, it is worth discussing this with a paediatrician. A single result at the edge of the normal range need not signal a problem, but consistent deviations deserve attention.
Percentile charts and BMI calculators for children are a screening tool and do not replace medical consultation. Assessment of a child’s development and any dietary recommendations should be carried out by a paediatrician.
Calculate your child’s BMI and check the percentiles: BMI calculator for children →